[IgM Antibody against IgG Antibody Binding to Antigen, So-Called Anti-Antibody, Reacts Mainly with F (ab')₂ Fragment Possessing Antibody Activity].

Objective: To develop enzyme-linked immunosorbent assay (ELISA) for measurement of IgM antibody (anti-antibody) against human IgG antibody that is binding to antigen, to distinguish anti-antibody from other antiglobulins, e.g. rheumatoid factor (RF), and to search their localization of epitope on IgG molecule.

Methods: We chose purified human IgG anti-dsDNA antibody from a patient with systemic lupus erythematosus as IgG antibody and calf thymus dsDNA as antigen. IgG F (ab')₂ fragment was obtained by digestion of the IgG with pepsin. Those IgG and IgG F (ab') 2 with anti-dsDNA antibody were reacted with pre-coated dsDNA and formed immune complex on ELISA plate. On the other hand we prepared ELISA plate on which those IgG and IgG F (ab') 2 were coated directly for measurement of IgM RF and IgM antihinge antibody. Twenty-three sera from patients with rheumatoid arthritis were tested.

Results: Correct IgM anti-antibodies were obtained after subtraction of absorbance of IgM anti-dsDNA anti- bodies. Remarkable correlation between IgM anti-antibodies obtained by using whole IgG and those by using IgG F(ab')₂(n=23, r²=0.914, p=1.11X10-12). There were correlations neither between IgM antiantibodies and IgM RF(n=23, r²=0.001, p=0.889) nor between IgM antiantibodies and IgM antihinge antibodies (n=23, r²=0.063, p=0.249).

Conclusions: IgM molecule with anti-antibody specificity seems to be different from IgM RF and IgM antihinge antibody. Moreover, epitope recognized by IgM anti-antibody seems to localize on IgG F (ab') 2 but not on hinge region. [Original].